8 Dr. ('. rhnv. 7/,/v.s/ /,/,//;/,* an 



by using two coil combinations we get a better idea of the order of 

 :uTur;icy of the observations ;nnl h;ive some check mi the constancy 

 of the smaller coil resistances. 



As a rule, each fixed point determination depended on at least 

 iiliservations Thus, in ice we might have a balance a^ain-' 

 ('!>!'. next against CDGH, and finally against CDF again. By com- 

 paring the readings in the first and third observations one can make 

 reasonably sure that the ^thermometer has reached the tme tempera- 

 ture. This is a very necessary precaution, especially in sulphur point 

 observations. 



When only three observations were taken, only half weight was 

 allowed to the first and last, so as to give equal weight to the two 

 combinations of coils. 



1 1 was originally intended to take a complete set of " fixed point " 

 observations, ice, steam, and sulphur points, with each thermometer 

 once a month. The regular observations have been in reality less 

 frequent than was intended, and the intervals between them more 

 irregular. In addition to the regular oljservations there have l)een a 

 variety of special occasions on which ice and steam point determina- 

 tions have been made, more especially with K;. This thermometer 

 has been used in comparing occasional high-range mercury thermo- 

 meters. 



Sources of CJuinge or Error. 



$ 7. Before considering the main question, I propose discussing a 

 number of possible sources of change in the readings obtained with 

 platinum thermometers. An error of invariable amount from a source 

 external to the thermometer is perhaps not immediately germane to 

 this inquiry, but the invariability of error is a very difficult thing to 

 ensure. 



Excluding observational error or defects, such as moisture in the 

 tube, which an experienced observer is likely to detect, I would chronicle 

 the following sources of trouble : 



1 . Change in the resistance of the platinum spiral ; 



2. Change in the thick platinum wires connecting the spiral to the 



thermometer terminals, or in the compensating loop inside the 

 tube; 



3. Change in the relative resistance of the leads, or in the relative 



resistance of the proportional arms of the box ; 



4. Change in the box coils or in the bridge wire ; 



5. Faulty action of the contact piece ; 



6. Shift of the bridge centre ; 



7. Thermo-electric currents ; 



8. Heating due to the battery current ; 



